In the past, in Si and other semiconductor crystals, the practice has been to prevent the drop in device characteristics due to metal impurities by using ion implantation, laser irradiation, formation of a film of different materials, etc. to form strain regions and thereby form gettering sites of impurities.
As opposed to this, in the case of an SiC crystal, the coefficient of diffusion of impurities is extremely small, so at the heat treatment temperature (at the maximum, 1200° C. or so) which is normally used with Si crystals, it has been believed that heat diffusion substantially does not occur (NPLT 1). For this reason, the art of gettering of SiC has not been considered much at all.
When forming an ohmic electrode at an SiC semiconductor device, it is necessary to form an n+ layer at the surface of the SiC substrate, use vapor deposition or sputtering etc. to form a metal film for electrode use, then perform heat treatment (800 to 1000° C. or so) for alloying. This heat treatment is performed for alloying the SiC surface and lowering the φs for achieving the ohmic contact conditions of the electrode (φm<φs:φm=work function of metal, φ=work function of semiconductor). Further, this is sometimes performed for adjusting the barrier height of a Schottky contact.
That is, it is essential to form a reaction layer of the electrode metal and SiC (alloyed layer), but the inventors newly discovered that through the formation of this reaction layer, the electrode metal diffuses into the SiC crystal. The metal, in particular a transition metal, among these Ni with the largest atomic number etc., freely diffuses in the SiC crystal. In particular, they newly discovered the problem that when forming a high doping layer for forming an ohmic electrode by epitaxial growth, the electrode metal which is taken into the crystal ends up freely diffusing without obstruction.